This invention relates to color television, and more particularly to a method and apparatus for an improved color television transmission that is free of chroma crawl and compatible with the standards of the National Television Systems Committee (NTSC).
The term "chroma crawl" refers to the effect of a jagged vertical edge of an image due to the color encoding and decoding process. The effect is most apparent at a border between contrasting colors. When viewed in a pattern of a calibrating color bar pattern, the jagged edge resembles a zipper. It will always be present in every video scene as a color distortion, and be quite noticeable, particularly in still pictures if viewed at a close distance. What is more disturbing is that the "teeth" of the "zipper" progress (crawl) upwardly on the screen. This can make the zipper effect very noticeable, even when viewed from a distance, because the color distortions produced on the image appear as waves, such as waves in the fabric of an article of apparel having a pattern of thin stripes, or some other pattern having parallel lines with a significant vertical component over substantial distances.
The NTSC color system transmits encoded color information on a subcarrier derived in the following way. First I and Q signals are derived from red (R), green (G), and blue (B) camera signals in a color matrix according to the following equations: EQU I=0.6R-0.28G-0.32B EQU Q=0.21R-0.52G+0.31B
Alternative color matrixing is given by the following equations: EQU I=0.27(B-Y)+0.74(R-Y) EQU Q=0.41(B-Y)+0.48(R-Y)
where Y is the luminance signal given by the equation: EQU Y=0.30R+0.59G+0.11B
Then the I and Q signals are modulated by the subcarrier in phase for the I signal and in quadrature for the Q signal. At the receiver, the phase encoded I and Q color signals are decoded by synchronously demodulating the inphase and quadrature components to recover the I and Q color signals. A receiver matrix then recovers the R, G and B signals from the recovered I and Q signals, using Y signals, if necessary.
The monochromatic television standard already in place required a frame of 525 lines at a rate of 30 Hz using interlaced fields of 262.5 lines per field at a field rate of 60 Hz. This requires a horizontal scan rate of 15,750 Hz which produces a monochrome signal having components at integral multiples of the horizontal scan rate. In order to transmit the color subcarrier in the 6 MHz band alloted to a television channel by the FCC, it was necessary to select a color subcarrier frequency high in the picture carrier single sideband, but sufficiently below the sound carrier to avoid interfering with the audio signal, which is 4.5 MHz above the picture carrier. That placed the color subcarrier at about 3.6 MHz above the picture carrier. To avoid interference with the monochromatic video signal even that high in the single sideband of the video signal, the color subcarrier frequency was chosen to "interleave" in the gaps of the integral multiples of the line-scanning rate where there is no great amount of video information. These gaps occur at odd multiples of one-half the line-scanning rate.
This need to interleave the color subcarrier sidebands with the picture carrier single sideband thus required the color sub-carrier to be at some odd multiple of one-half the line-scanning rate, which is a multiple of 455 for the color subcarrier to be placed high in the video single sideband, but below the audio carrier. This odd multiple of half the line frequency also results in a minimum beat frequency with the audio carrier.
The precise color subcarrier chosen was 3,579,545 Hz to have 227.5 cycles of the color subcarrier per line scan, thus inverting the color subcarrier phase 180.degree. for each successive line. That would tend to cancel any effect of interpreting luminance transitions of color in the demodulation process, but instead produced the effect described above called "chroma crawl." This is because each field has only 262.5 lines. The half line at the end causes the phase of the color subcarrier to be shifted only 90.degree. for the beginning of the next field. So, in a sequence of four successive fields of two frames, the phase of the subcarrier is 0.degree., 270.degree., 180.degree. and 90.degree.. This is so because 227.5 Hz/line times 262.5 lines equals 59,718.75 cycles per field, and 119,437.5 cycles per frame. Consequently, on a field to field basis, the luminance transitions interpreted as color will appear, albeit to a lesser extent, and it will appear to crawl up the television screen.